service: Drop a redundant 'const'

This was generating a warning from gcc.
3 jobs for drop-depcrecated in 3 minutes and 27 seconds (queued for 16 seconds)
Status Job ID Name Coverage
  Test
passed #73314
ubuntu-16.04

00:01:46

passed #73315
ubuntu-17.10

00:03:21

passed #73316
ubuntu-18.04

00:02:59